Mmm, Potassium

Even Freud would concede that a runner who finishes a marathon with bananas on the brain is not thinking impure thoughts. There's a simple, physiological reason for the water-and-bananas combo that's become a staple in almost every postrace recovery tent: Water hydrates and bananas supply potassium.

Potassium is a mineral that works with sodium (also a mineral) to balance the fluids and electrolyte levels in your body. And since steady fluid levels help to regulate your heartbeat and prevent muscles from cramping, potassium is of particular importance to runners. "Think of it as the gatekeeper for fluid movement in and out of the body's cells," says Lisa Dorfman, R.D., a sports nutritionist at the University of Miami's athletic department. Most of the sodium in your body is stored outside your cells, while most of the potassium is stored within. Yet because of their different concentration levels, potassium constantly wants to get out and sodium wants to get in. The transfer of these two crucial minerals in and out of the cells--the "sodium-potassium pump"--comprises 20 to 40 percent of an adult's resting energy expenditure.

Put that adult in motion, running, and studies have shown that he or she will finish a marathon with more potassium outside his or her cells than inside. That's why you feel weak, your legs might start to cramp, and you may begin to feel bloated. But thanks in part to that unoriginal (albeit useful) banana and water foisted upon you at the finish line, the imbalance returns to normal in about an hour.

Although potassium's main job is maintaining fluid and electrolyte balance within the body, it may also protect against heart disease. In the definitive 1997 study called Dietary Approaches to Stop Hypertension (DASH), conducted by the National Heart, Lung, and Blood Institute, volunteers who took in 4,700 milligrams of potassium per day through a well-rounded diet that was loaded with fruits, vegetables, whole grains, and low-fat dairy products, plus some fish, poultry, nuts, and beans, lowered their blood pressure in just two weeks. "Potassium can act as a diuretic," says Leslie J. Bonci, R.D., director of the sports-medicine nutrition program at the University of Pittsburgh Medical Center. "This is helpful because it gets rid of extra fluid and decreases the pressure forced through the cardiovascular system."

Since potassium plays such a key role in hydration, runners might be worried about taking in enough. Relax. Potassium is found in so many common foods that supplementation is rarely necessary to meet the Daily Value of 3,500 milligrams. High-mileage runners may want to take in closer to 4,700 milligrams (you can safely ingest three times that per day), but these amounts are easy to attain by eating a varied diet that consists of lots of fresh, unprocessed foods. That's because potassium is abundant in all living plant and animal cells. The cells remain intact--along with their potassium content--until food is processed, making whole foods the best source. "Just one regular carrot, or six baby carrots, has about 300 milligrams," says Bonci. "What I tell athletes who are already paying extra attention to their carbohydrate and protein intake, is to simply choose carbs and proteins that are also rich in potassium." Bonci's five favorite potassium sources are: a four-inch-long baked potato, which contains about 800 milligrams; a banana (500 milligrams), eight ounces of milk (450 milligrams), eight ounces of yogurt (400 milligrams), and a nectarine (300 milligrams).

While it's hard to become potassium deficient, it's not impossible. Dorfman cautions that some college athletes who train consistently in hot, humid weather and who eat a typical college diet--lots of fast foods that are high in sodium and low in potassium--have exhibited signs of potassium deficiency such as pronounced muscle fatigue. So it's best to concentrate on eating a wide variety of whole foods every day to keep your potassium levels optimal. That and eating that banana in the recovery tent.

Island Vegetable Soup can be served warm or cool and contains 1,486 milligrams of potassium per cup. The recipe below, reprinted from The Tropical Diet, by Lisa Dorfman, R.D. (Food Fitness International, Inc., 2004), serves eight.

Steam the cassava and yams over high heat until tender. Remove and set aside.

Heat cooking spray in a sauté pan, then add onion and pepper. Sauté until fragrant, about three minutes.

Add chives, garlic, ginger, and tomatoes, breaking the tomatoes with the back of spoon. Add the basil and thyme and cook covered for an additional 10 minutes over low heat. Season with salt and freshly ground black pepper.

Stir in steamed cassava and yams, plantain, tomatillos, and combine. Add chicken broth and cook for one to two minutes more. The consistency should be like a stew.

Each one-cup serving has: 252 calories; 59 grams of carbohydrate; five grams of protein; one gram of fat.

If you don't like the most popular potassium-packed fruit, these foods all have more than 225 milligrams of the mineral per half-cup serving:

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